In U.S. Pat. No. 3,981,481 granted Sept. 21, 1976 filed Oct. 8, 1974, a railway tank car bottom outlet valve assembly is disclosed in which the outlet chamber and a portion of a bottom operator for the valve closure drop off upon impact to the outlet chamber. In FIGS. 5-18 of this patent, movement of the valve closure between open and closed positions requires that a depending portion of the valve closure engage a retainer or spider to prevent rotation of the valve closure and allow the valve closure to move between open and closed positions.
While the construction shown in the patent represented a significant advance in the art in providing an outlet chamber which would shear off upon impact and prevent the lading from escaping during such an impact to the outlet chamber, the relatively thick mounting flange required in this construction necessary to mount both the valve seat and the spider or retainer to prevent rotation of the valve closure necessitated a mounting flange which extends more than one inch below the bottom surface of the tank bottom.
AAR and DOT regulations concerning projections extending below the tank bottom prohibit the mounting flange from extending more than one inch below the tank bottom without additional shear-off protection.
Another problem with the construction illustrated in the '481 patent is that the projections extending downwardly from the valve closure to prevent rotation of the valve closure also function to stop vertical movement of the valve closure in the full open position. In order to have satisfactory unloading rates, the amount of travel of the valve closure and the depending extensions must be of the order of 11/2-2 inches. If these valve closure extensions were to be reduced in vertical extent sufficient to have a mounting flange extend no more than one inch below the tank bottom, the amount of available travel of the extensions would be less than one inch. This would result in too low lading unloading rates and loading rates for commercial acceptance.
It is unpleasant for the attendant and to some extent a safety hazard for the attendant to be located within the tank during installation. It therefore is desirable for the valve closure to be installed from the bottom of the tank.
In subsequent work developed by Assignee disclosed in U.S. Pat. No. 4,198,032 a bottom operable tank car valve assembly is disclosed which can be installed from the bottom of the tank without an attendant located within the tank. In this assembly a retaining ring is provided which supports and holds in place a valve seat for a lading valve closure.
During assembly the valve closure is placed upon the valve seat. A key is wedge into a slot in the valve seat. This assembly is lifted up into the tank until a valve seat shoulder abuts a shoulder of a slot in a tank mounting flange. The valve seat is rotated until the key aligns with a slot in the tank mounting flange. When the operator is thus supporting the valve seat and valve closure, a retaining ring is lifted until it abuts a ledge on the valve seat and another shoulder on the valve seat to support the valve seat. The retainer is then rotated until fastener openings in the ring align with fastener openings in the mounting flange and a fastener is started into the threads in the mounting flange.
This is a difficult and awkward operation for an attendant situated below a railway tank car. The key is sometimes lost. Even if the key is not lost, aligning the ring with fastener openings in the mounting flange is difficult and is particularly unpleasant in inclement weather.
Thus, this subsequent development of the retainer with the key and key slot has not been found to be entirely satisfactory.